Polyester resins, such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), and copolyether ester block copolymer elastomers have, for many years, been commonly used to form articles of all sizes and shapes. Each group of materials has its own advantages and drawbacks. More specifically, the PBT and PET polyester resins generally yield products having high rigidity and good resistance to many corrosive chemicals. In contrast, copolyether ester elastomers are renowned for their flexibility, resistance to fatigue and soft touch characteristics. Many mechanical and electrical components comprise pluralities of functional parts requiring a combination of such rigid and flexible materials.
Various vibration-damping polyester compositions are known. Resins that have good sound damping properties (measured as a high tan delta damping factor) generally have insufficient rigidity to be useful in many applications such as relay caps, fuse boxes, relay boxes or the like. Tan delta (or “dynamic loss tangent”) is a measure of sound damping properties that is measured using a Dynamic Mechanical Analyzer which provides a quantification of a material's energy absorption expressed as a numerical value that is not associated with any units.
It has been proposed to blend a hard polyester resin with a soft elastomer having a high tan delta in order to produce molded parts that exhibit good sound damping properties. For example, JP-A 32 634 454 discloses a vibration damping polyester composition comprising a crystalline thermoplastic polyester, a polyester elastomer comprising polyester hard segments and polyether soft segments, and reinforcing fibers. However, as reported in U.S. Pat. No. 5,814,696, the foregoing composition undergoes a sharp reduction in rigidity due to the inclusion of the elastomer and cannot have a large vibration-damping because of the difficulty of increasing the amount of the elastomer to be added in practical use.
This U.S. Pat. No. 5,814,696 proposes to overcome the stated problem with a polyester resin composition having vibration-damping and noise suppression properties obtained by blending an aromatic polyester resin with a polyester block copolymer resin which consists essentially of polyester soft blocks and polyester hard blocks, optionally with a fibrous filler. This is said to achieve a product with a flexural modulus of at least 1,500 MPa using a polyester block copolymer resin that has a maximum value of the tan delta in a range of temperatures between −20° C. and +40° C., preferably between −15° C. and +20° C., without loss of rigidity.
It is known that copolyether ester elastomers maintain a tan delta damping factor over a wide temperature range, meaning that they exhibit good noise damping properties over this wide temperature range which is important for many end use applications. However, exploiting this property has been handicapped by the problems of associating this material with a more rigid material. The composite materials proposed so far are either limited in their range of temperature application, expensive to produce, or both. Generally speaking it can be said that the acoustic damping of these composite molded materials is a function of the inherent frequency of the material which is dependent on the E-modulus. This leads to limitation of the degree of damping, in particular under extreme acoustic conditions as encountered with relay caps.
Another approach to combining rigid polyester resins and flexible elastomers is the use of multicomponent thermoelastic elastomer compositions, for example a composition containing PET or PBT, an epoxy group containing ethylene copolymer, specific polyfunctional compounds and a block copolyether ester elastomer, as disclosed in U.S. Pat. No. 5,405,909.
Alternatively, it has been proposed to make parts like relay caps from two separate materials with greatly differing E-modulus. This is exemplified by U.S. Pat. No. 5,354,532 which proposes making composite molded articles having two discrete regions. One region is of a relatively highly rigid resin such as PET or PBT, the other region is of a relatively flexible copolyether ester elastomer, and the separate resins are molded to produce an interfacial bond. This produces bodies with a surface layer of the elastomer having good sound damping properties where most needed so as to provide better sound damping properties than is possible with parts made of one material. This is because vibrational energy is converted into heat in the flexible elastomer. However, co-molding of the separate components is relatively expensive and, as the soft component lays over the molded part, its poor temperature stability remains problematic.
In summary, there remains a need for a molded composition having excellent vibration damping and noise suppression properties over a wide range of temperatures and that can be manufactured inexpensively.